In the field of magnetic recording using a head and media medium, further improvement in performance of the magnetic recording medium and the magnetic recording head is required along with the increase in the recording density of the magnetic disk device.
The magnetic recording medium is a discontinuous medium in which magnetic grains are gathered, and each magnetic grain has a single domain structure. In this magnetic recording medium, one recording bit is configured by multiple magnetic grains. Consequently, to increase recording density, the magnetic grains must be made smaller, and the unevenness in the boundary between neighboring recording bits must be reduced. However, when the magnetic grains are made smaller, the problem arises that the thermal stability of the magnetization of the magnetic grains drops accompanying the decline in the volume of the magnetic grains.
As a countermeasure to this problem, enlarging the magnetic anisotropy energy Ku of the magnetic grains has been considered, but this increase in Ku causes an increase in the anisotropic magnetic field (coercive field) of the magnetic recording medium. In contrast to this, the upper limit of the recording magnetic field intensity by the magnetic recording head is virtually determined by the saturation magnetic flux density of the soft magnetic material composing the magnetic core in the head. Consequently, when the anisotropic magnetic field of the magnetic recording medium exceeds the tolerance determined from the upper limit of this recording magnetic field intensity, recording on the magnetic recording medium becomes impossible.
At present, as one method of resolving this kind of thermal stability problem, energy assisted recording has been proposed in which a magnetic recording medium formed with a magnetic material having a large Ku is used and the effective recording magnetic field intensity is lowered by supplementally applying energy to the medium at the time of recording. A recording method that uses a microwave magnetic field as this supplemental energy source is called Microwave Assisted Magnetic Recording (MAMR), and research and development is being promoted for practical use.
In microwave assisted magnetic recording, by applying a microwave magnetic field in the in-plane direction of the medium having a frequency corresponding to an effective magnetic field (Heff) applied to the magnetization of the recording layer of the magnetic recording medium, precession of magnetization in the recording layer is stimulated, and the recording capability of the magnetic recording head is assisted.
As one example of a magnetic recording head using the microwave assisted magnetic recording method, as shown in FIG. 14, a magnetic recording head that includes a main magnetic pole 6′, for generating a recording magnetic field to be applied to the magnetic recording medium, a trailing shield 81′, and a spin torque oscillator (STO) 10′, which has a multi-layer structure of magnetic films and is provided in a write gap between the main magnetic pole 6′ and the trailing shield 81′, has been proposed (for example, see U.S. Pat. No. 9,001,465 and JP Laid-Open patent application No. 2005-25831). The spin torque oscillator 10′ is an element that receives spin transfer torque. The magnetization of the spin torque oscillator 10′ fluctuates while precessing under the influence of the spin transfer torque. The magnetic field emitted from the spin torque oscillator 10′ interacts with the recording magnetic field, to improve recording performance. For example, the spin torque oscillator 10′ can generate a microwave magnetic field in the in-plane direction through the self-oscillation thereof. By superimposing the microwave magnetic field and the recording magnetic field on the magnetic recording medium, precession of the magnetization of the recording layer is induced, and magnetization in the perpendicular direction in the recording layer is reversed.
In such a magnetic recording head, it is considered that the recording properties of the magnetic recording head can be improved by increasing the intensity of the magnetic field (hereafter referred to at times as the “assist magnetic field”) emitted from the spin torque oscillator 10′. By increasing the total film thickness of the spin torque oscillator 10′, which is configured by a laminated body including a ferromagnetic layer, the intensity of the assist magnetic field can be increased. However, to increase the total film thickness of the spin torque oscillator 10′, it is necessary to increase the length in the down-track direction of the write gap between the main magnetic pole 6′ and the trailing shield 81′. When this length of the write gap is increased, the main magnetic pole 6′ and the trailing shield 81′ are separated from each other, and the magnetic field gradient of the recording magnetic field emitted from the main magnetic pole 6′ and applied on the magnetic recording medium is greatly reduced. As a result, even if the assist magnetic field, the intensity of which has been increased, interacts with the recording medium, there is a concern that desired recording properties in the magnetic recording head may not be obtained. Consequently, it is required to improve the intensity of the assist magnetic field from the spin torque oscillator 10′ provided in the write gap, while reducing the length of the write gap in the down-track direction.